NIST-on-a-Chip: Atomic Vapor

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NIST scientists anticipate that as many as five or more basic quantities can be measured with quantum accuracy using vapor-cell technology.

Perhaps no single technology more fully embodies the goals of NIST-on-a-Chip than the use of tiny atomic vapor cells for realization and measurement of quantities ranging from time and length to electric and magnetic field strength, current, voltage, temperature, and even electrical fields. Experimental devices for each of those applications, and more, are now in development.

Because their operation depends on fundamental physical constants and well-known quantum phenomena -- specifically, atomic transitions in trapped vapors of alkali atoms or molecules -- most of the devices will be intrinsically self-calibrating. Because their dimensions are small, they can be made at comparatively low cost using the familiar tools and techniques of microelectronics and microelectromechanical systems (MEMS) fabrication, and thus more readily commercialized.

When eventually deployed, they will hold the promise of bringing high-accuracy, SI-traceable measurements to applications where their conventional counterparts cannot go, such as battery-powered mobile computing, telecommunications and GPS devices, miniaturized sensors and calibration standards for the factory floor or defense use, small-scale space satellites, and more.

The vaporized atoms or molecules, contained in glass cells, are manipulated and interrogated by low-power lasers. Perhaps the best-known example of this technology is the world’s first chip-scale atomic clock (CSAC), invented at NIST in 2004 and commercialized in 2011. The inner workings of the initial prototype take up 9 mm3, about the size of a grain of rice. It uses laser beams traveling through a cesium vapor to identify the frequency of a specific atomic transition between two discrete energy states.

That frequency, detected photonically and locked onto by a feedback system, is exactly fixed by the laws of quantum mechanics, so it can be used as a standard for correcting a time signal. (For a technical overview of this and related work, click here.)

In a generic atomic vapor cell, a laser (at top, white) sends light through a population of atoms (blue-gray dots) trapped in an airtight cell (dark gray). Properties of the light change as it passes through the atoms, and the result is detected by a sensor (red spot on yellow rectangle).

Atomic vapor-cells are typically very small: For many applications, experimental cell size has already shrunk below 1 mm3, and NIST scientists hope to eventually reduce the dimensions of some to less than 100 micrometers -- the width of a human hair -- on a side.

But small size is only one advantage. The devices also will have unprecedented portability and consume very little power. (The CSAC, for example, operates at 120 mW or less -- lower by a factor of about 30 than similar devices.) Moreover, if past trends continue, manufacturing costs are expected to be roughly an order of magnitude lower than those for conventional instruments. Perhaps most important, no matter what quantities they are measuring, all the devices have outputs in the form of specific frequencies of light; and frequency can be determined to higher accuracy than any other measurand.

NIST researchers are now at work on atomic vapor-cell devices that could measure, or serve as standards for, units of the following properties. Click on each link to see details.